1. Field of the Invention
The present invention relates to a flat display screen. It more specifically applies to the assembling of two plates forming the bottom and the surface of the screen, and between which an internal space isolated from the outside is provided.
2. Discussion of the Related Art
Conventionally, a flat screen is formed of two external generally rectangular spaced plates, for example made of glass. One plate forms the screen surface while the other forms the bottom of the screen generally provided with the emission means. These two plates are assembled by means of a sealing gasket. For a field effect display (FED) or a microtip display, or for a vacuum fluorescent display (VFD), vacuum is made in the space separating the two glass plates.
FIG. 1 schematically shows in cross-sectional view the conventional structure of a microtip screen.
Such a microtip screen is essentially formed, on a first substrate 1, for example, made of glass, of a microtip cathode and of a grid. In FIG. 1, the cathode/grid assembly is designated with common reference 2. Cathode/grid 2 is placed opposite to a cathodoluminescent anode 3 made on a second transparent substrate 4, for example in glass, which forms the screen surface.
Cathode/grid 2 and anode 3 are made separately on the two substrates, or plates, 1 and 4, then assembled by means of a peripheral sealing gasket 5. An empty space 6 is formed between the two plates 1 and 4 to enable the circulating of the electrons emitted by the cathode towards anode 3.
The assembly of plates 1 and 4 is conventionally performed as follows.
First, spacers 7 for defining empty space 6 are glued onto cathode/grid 2. These spacers 7 are generally formed of glass balls regularly distributed so that spacing 6 between plates 1 and 4 is constant.
Then, cathode/grid 2 is submitted to a thermal vacuum processing having the purpose of degassing the cathode and evaporating the glue of spacers 7. A similar processing, not necessarily performed in vacuum conditions, is applied to anode 3.
According to a first conventional assembly method, a pumping tube 8 is provided on the free surface of first plate 1. This tube 8 is, for example, in glass, and is sealed, by one of its open ends, above a hole made in plate 1 to establish a communication with space 6. This tube 8 will be used in particular to create vacuum in space 6. Tube 8 is placed at a corner of plate 1 outside its useful surface. Then, a seal 5, for example a fusible glass cord, is deposited on the circumference of plate 1 or 4. Both plates 1 and 4 are then assembled by being pressed against each other, cathode/grid 2 facing anode 3, and this assembly being submitted to a temperature enabling the melting of cord 5. The structure obtained is then submitted, via tube 8, to a hot pumping which has the function of degassing space 6. This degassing is necessary, in particular, because of the gases emitted by fusible glass cord 5 during the sealing of the plates. Tube 8 is then closed at its free end after introducing therein a getter 9. The function of getter 9 is to absorb any contamination likely to appear during the subsequent operation of the screen. In FIG. 1, tube 8 has been shown as closed, that is, once the screen is completed.
A disadvantage of such an assembly method is that the necessary use of a pumping and getter hosting tube causes a bulk problem for the screen.
To overcome this disadvantage, methods of vacuum assembly of a microtip screen have been provided.
FIG. 2 is a lateral view illustrating a conventional method of vacuum assembly of a microtip screen.
In such a process, plates 1 and 4 are maintained in vacuum conditions since their respective degassing thermal processings and are placed against each other with an interposed fusible glass seal 5'. The peripheral fusible glass seal, or frame, 5' exhibits, before assembly, irregular surfaces of contact with plates 1 and 4. The whole is maintained under pressure, for example, by means of pliers 10. The whole as shown in FIG. 2 is then submitted to a temperature which enables the softening of frame 5' while remaining lower than its melting temperature. At such a softening temperature, the leaks 12 linked with the irregular surfaces of frame 5' are meant to enable an outlet of the gases emitted by the fusible glass frame inside the screen before the glass sealably assembles the two plates during its melting. This melting of frame 5' is obtained by bringing the whole to a temperature higher than the melting temperature of frame 5', which then causes, under the effect of the pliers, a crushing of frame 5' and the assembly of the screen, the distance between the plates being fixed by the spacers. A disadvantage of such a method is that it does not enable to completely eliminate the gases emitted by cord 5'. Indeed, the fusible glass continues to degas when brought from its softening temperature to its melting temperature.